Over the last decade, I’ve continued to gravitate more and more toward Unix and the command line while many of my colleagues have gravitated toward GUI applications. This post is not about the pros and cons of GUI vs. CLI or how one is better than the other. Suffice it to say, I live with both, but spend a lot of time working in terminals.

Today we focus on the prompt. Many people leave the prompt set to the default for their distribution, which may be fine, but in my 20 years of Unix use, I’ve found some nice tweaks that make my life easier. I share them here because you may find them useful as well.

The default prompt for a Red Hat Enterprise Linux system looks like the following:

[smj@athena bin]$

This gives me an idea of which user I’m logged in as (smj), which server I’m logged in to (athena) and the current working directory (bin). Unfortunately, in the example above, my current working directory is actually /usr/local/bin. Based on the default prompt, I cannot tell if I’m in /usr/local/bin, /opt/bin, /usr/bin, /bin, or even /home/smj/bin. This presents a problem considering how often much of the Unix directory structure repeats itself.

Another issue I have is that I’m pretty sure I’m logged in to the bash shell, but can’t really be sure. I’ve had to endure many shells in my career, from sh to bash to csh to tcsh to ksh and some I can’t even remember because they appeared so infrequently.

So, to address these problems, I spent time trying to find a prompt that would provide enough information to be useful, while working across several platforms and shells.

My current bash prompt looks like so:

18:30:33 smj@athena:/usr/local/bin
bash $ -->

My current tcsh prompt looks like so:

18:40:01 sjone@procyon:/usr/local/bin
tcsh % -->

There is no color in either prompt. This avoids any issues between different terminal color schemes or terminal types. It works equally fine in the Linux console and hpterm.

You may be asking, why does he need all of this information in the prompt? Let’s review each part of my prompt.

Newline at start

Consider the following text from an open terminal with a prompt containing no information:

Can you quickly glance out it and tell me which commands have been executed? I need some separator on the screen to indicate the individual command executions so I can tell what happened. Without a separator, it looks like one big garbled mess, so I opted for a newline between the prompt and the command executions.

Now I can clearly see that the top is the output of some command that has scrolled off the screen, but the cat, netstat, and gvim commands came next. It’s not that I can’t figure out what commands were executed (or even look at the history), but that when I’m comparing commands and output to each other, I need to be able to quickly see which sections of output belong to which commands. For me it’s a legibility and time-saving measure.

Time last command exited

At the beginning of the prompt is a timestamp.

18:30:33 smj@athena:/usr/local/bin
bash $ -->

This timestamp serves as a “poor man’s time command” letting me know (roughly) just how long the previous command took to run. It also serves as a way to keep track of times if I have a meeting or other engagement coming up. I put it in front, so when I see the prompts in sequence I can compare them to one another.

Username

Next is the username.

18:30:33 smj@athena:/usr/local/bin
bash $ -->

I don’t have the same username for each account. I also want to know if I’m running as root or a regular user. Sometimes that forces me to notice that I’m root before I do something dangerous. With one terminal up, this is not a problem. When I’m switching between five, it becomes increasingly important to keep each straight.

Server hostname

And equally useful to the username is the hostname of the server to which I’m connecting.

18:30:33 smj@athena:/usr/local/bin
bash $ -->

With the hostname I can tell different terminal sessions apart and ensure that I don’t make a mistake of typing the wrong command on the wrong server.

Full Path

As noted above, I like seeing the full path in my prompt. This way I can tell where I am on the server.

18:30:33 smj@athena:/usr/local/bin
bash $ -->

I also have a newline inserted after the path because a long path can scroll pretty far across the screen and some terminals have an issue including both the command and a long prompt on the same line.

The whole username@hostname:path exists so I can easily copy and past this string into an SCP command for moving files between servers. This is easier than typing scp myfile username@hostname:path by hand, especially considering hitting <tab> doesn’t work to expand the path for the server you are copying to.

Shell Name

This was one of the last things I added. I use a lot of bash; a LOT OF bash, but occasionally I have accounts (thank you ODU) that are tcsh, or I need to use an ancient server running csh or sh. I wanted to ensure that I could readily identify the shell as I switch between systems, so I have the prompt state which shell is running.

18:30:33 smj@athena:/usr/local/binbash $ -->

This means that I actually have login scripts for each shell that attempt to create as close as possible an approximation of the bash prompt. For example, if I’m suffering through an actual Bourne Shell session, I only get this:sjone@E-2104-13.cs.odu.edush $ -->

For tcsh, I get this:
18:40:01 sjone@procyon:/usr/local/bintcsh % -->

The point is that the shell is identified on the line on which I’m typing commands. This way I will know about the shell so I can change my syntax for items, such as redirecting stderr to stdout, when using a shell other than bash.

I do not control all of the servers I log into. Even though I may prefer bash, administrators on other systems have everyone default to tcsh or other shells. I try to ensure that my login prompt reflects this difference, especially when I’m working across several machines.

"The Arrow"

I added the arrow (–>) with an additional space afterwards so I can separate the prompt from the command that has been run. I find this to be useful, much like the newline between prompts, when looking at the output from a series of commands. It allows me to separate the commands I run from their output.

No color

Finally, I work across multiple systems with different terminal emulators, some of which I cannot choose for myself. At work I use PuTTY or Attachmate Reflections. At home I prefer the Mac OS X terminal program, or < a href="https://help.gnome.org/users/gnome-terminal/stable/">gnome-terminal, but I don’t always get to use my home computer for all of my projects. Sometimes, I don’t even get to configure the terminal how I would like!

Imagine that I had selected a color scheme using blue. Now imagine that same blue with a black background. Was that even legible? If cannot change the background to white or gray then I have to strain to read that text. Also, consider yellow on a white background. If I cannot reliably control my background colors, I really can’t select any meaningful foreground colors either.

Hence, I put not color into my prompt.

Setting up the prompt

To generate my prompt, I set the PS1 variable to the following value in my .bashrc file:
PS1='\n\t \u@\h:\w\nbash \$ --> '

The \n provides the newline, the \t provides the time, the \u provides the username, the \h the hostname, the \w the full path, and the rest is just text.

TCSH is not that much different:
set prompt = "\n%P %n@%m:%~\ntcsh \% --> "

Here, the %P provides the time, the %n provides the username, the %m the hostname, the %~ the full path (substituting ~ for my home directory), and the rest is just text.

Other shells do not have these fancy variables for telling time or providing other information, so for those shells I just set them with a text string providing as much meaningful information as I can get. I even have a custom Windows Command Prompt I use when I’m able to set it myself.

Summary

I hope you have found this article a useful resource. Though I realize not everyone is in the same situation I am, having to adapt to many differing environments, I hope that you can take away something useful from this discussion about the importance of a prompt on the command line.

It has been very fun. Because this was a small academic project, I’ve had unprecedented freedom in my processes and technologies for getting the job done. It has worked well for me, leading me to find the best solutions possible.

I have published the results of my further analysis as a Tech Report. Publishing to arXiv is a process I will need to document at a future date, but it has been rewarding to see months of work documented for posterity.

Lately, over the past few years, I’ve been pursuing my Master’s Degree in Computer Science from Old Dominion University. It’s been a crazy ride. Some of the courses have merely been a review of information I already knew. Others were some of the most difficult in my life.

I’ve wanted SELinux with my Red Hat/CentOS/Scientific Linux/Fedora servers since Red Hat ironed out the details around Fedora 6. The benefits are that someone breaking into one of the services is “sandboxed” and it limits the damage they can do to the system.

The drawbacks are that often items don’t function as one would expect because you have not found the correct set of draconian rules to enable the functionality you are looking for, but Red Hat has worked to iron out a lot of that to great success in the last few years.

I changed my SELinux setting in /etc/selinux/config from “disabled” to “permissive” so I can start up services and monitor /var/log/audit/audit.log to determine which SELinux rules should be altered.

After rebooting my server and analyzing the logs, I used the fixfiles command to change the SELinux contexts to get rid of some errors and saw the following message on several files:

failed to change context of `/etc/resolv.conf' to `system_u:object_r:net_conf_t:s0': Operation not permitted

I assumed (wrongly) that this must be some issue with SELinux. I tried the chcon command and got the same message. I could run chcon fine on other files, so what was the problem?

I decided maybe something was up with the file inode, so I tried moving the file aside with the intent of filling it in with the proper data/permissions/etc. after. The cp and mv commands also returned with Operation not permitted.

The permissions on the file showed that the files were owned by root and root had write permission to the file:

-rw-r--r--. 1 root root 97 May 31 17:39 resolv.conf

Now this was less an SELinux problem and more an issue specific to these files. Some more goggling led me to the obscure lsattr command, which showed:

----i---------- /etc/resolv.conf

The i is the “immutable bit”. It is the Linux equivalent to Microsoft Windows’s “Read Only” checkbox. I had known about it, but could not remember the command to check for it.

I’m in. By in, I mean I have the MacBook Pro (it’s got Unix!), the iPad, the iPod, the iPhone, and the MacMini Server. For documents I still use Microsoft Office like the majority of the rest of the world, but for personal documents increasingly I’m using iWork because of some of nifty features, like autosaving and iCloud.

My first concern is space. You see, I’ve ripped about half of my CDs so far and I’m taking up 22GB of space on my iPod with music. As time goes on, I’ll probably reach 45GB in CD music alone. I’m in no danger of hitting my upper limit today, or next year even. My iOS devices (iPod, iPhone, iPad) all have 64 GB of space. I don’t put music on the iPad, preferring to use that device for movies and books, so the space taken up by the different forms of media shouldn’t interfere with one another.

I don’t have a lot of music with iTunes. I bought one album because it was only available there. If I continue to buy MP3s via Amazon, then I will continue to take up more and more space on my iOS devices.

I’m not sure where Moore’s Law will taper off in terms of more and more storage being available in these consumer devices. I doubt this trend will continue for consumer devices because more and more people are moving their storage to the cloud. My theory is that this will lead to fewer folks buying hard drives (whether they be magnetic or flash, it doesn’t matter), thus driving the demand down and the price back up again. With the price up, Apple (or Amazon, or Samsung, or whoever makes your portable music device) will not put as much storage into your portable device. This means that I will hit an upper limit on how many MP3s I can put onto my iOS device in the future. Apple wants me to use iCloud instead, because they see this future coming. More importantly, they are helping that future come.

So, the question is, do I continue to use Amazon’s MP3 store and manually move songs onto and off of my iOS device in the future, or do I just bite the bullet now and move to iTunes altogether? iTunes has the largest collection of music out there and I can always store a few MP3s for those artists and labels that I have to buy on Amazon.

Then there is an issue with preservation. The music on iTunes is (as most of you know) not stored in the MP3 format, but AAC (with various file extensions). As of 2009, it is supposedly DRM free. I opened one of my iTunes songs in VLC player, and it played fine, without asking me for a password like the DRM-full songs do. If this is the case, and AAC players are available in the future, then my concerns about preservation may be unfounded, as I will always be able to play the songs I’ve purchased.

iTunes is big, and with that many customers, I wouldn’t be surprised if AAC stayed around for a long time, so there should be no issue with finding an AAC player if iTunes goes away.

To recap:

Amazon MP3 Store

Apple’s iTunes

storage space issues

will require more and more storage on my iOS devices

handled by iCloud

preservation capability

MP3s are playable on many, many devices

AACs are no longer encumbered by DRM and are playable by a lot of open source software, meaning conversions are possible

So, there really is no reason I can’t just jump on the iTunes bandwagon and enjoy the benefits of iCloud.